Steel pickling process for application of one-coat enamel



United States Patent 3,156,578 STEEL PICKLING PROCESS FOR APPLICATION OF GNE-COAT ENAMEL Evan M. Oliver, Louisville, Ky., assignor to General Electric Company, a corporation of New York No Drawing. Filed May 24, 1961, Ser. No. 112,227 2 Claims. (Cl. 117-50) The present invention relates to a new and improved process for pickling steel surfaces and is more specifically concerned with 'a pickling process particularly adapted for the preparation of low carbon sheet steel for the application of a single finish coat of vitreous enamel.

Numerous treatments have been proposed in the art for the purpose of preparing a ferrous metal surface for the application to the surface of a single finish coat of vitreous enamel thereby eliminating the ground coat normally required to provide good adherence between the finish enamel layer and the base metal and to prevent the finish enamel coat from becoming contaminated by fusible color forming components of the ferrous base during firing of the finish coat. Many of these processes have not been completely satisfactory either because they required expensive treating materials or the use of special equipment or control means for maintaining or controlling the composition of the treating baths. Articles subjected to some of the processes cannot be reworked employing the same treating baths. The rework problem appears to be particularly troublesome in connection with those processes involving the use of phosphoric acid or other phosphates pickling baths.

It is an object of the present invention to provide an improved process for the pickling of a low carbon sheet steel.

Another object of the invention is to provide an improved and low cost process for the uniform pickling of sheet steel in preparation for the application of a single finish coat of enamel.

A further object of the present invention is to provide an improved process for pickling sheet steel by means of which the pickling rate and quality of the pickled article can be readily controlled.

Additional features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed disclosure.

The process of the present invention is particularly adapted to the pickling of a steel sheet or strip of low carbon steel or enamelling iron stock having a low carbon content in the neighborhood of .0l% and at least not exceeding about 0.15% carbon which has been shaped into the form of the designed finished article and which is intended to receive a single finish coat of enamel. The shaped article is first cleaned by use of an alkali cleaning bath, either spray or immersion, for the purpose of removing dirt, grease, oil or drawing compounds present on the surface of such products. While the invention is not limited thereto, a suitable alkali cleaning bath is one containing from about /5 to about 6 ounces of sodium phosphate per gallon of water and operated at a temperature of from 170 to 190 F. Following re moval of the cleaned article from the alkali bath, it is rinsed at least once with clean water to remove all traces of the cleaner. Thereafter, in accordance with the pres ent invention, it is pickled by means of a spray or immersion solution essentially containing, by weight, from 0.5 to 2.0% sulfuric acid, from 0.2 to 1.0% nitric acid, from .2 to 3% ferrous iron and from 0.1 to 0.5 ferric iron and is then rinsed with a dilute aqueous solution of sulfuric acid. The pickling bath, which is characterized by the fact that the sulfuric and nitric acid contents are substantially lower than those normally used in sulfuric or nitric acid pickling baths, is operated at a temperature of from 130 to 190 F. and the pickling or etching treatment is continued until the total amount of iron removed from the steel surface is between 1.5 and 4 grams per square foot of exposed surface. Preferably, the nitric and sulfuric acid proportions are one part by weight nitric acid to about 2.6 to 3.1 parts by weight of sulfuric acid. The pickled article is thereafter immediately rinsed in a dilute solution of sulfuric acid containing from about 0.5 to 5.0% by weight of sulfuric acid and operated at a temperature of from to 160 F. Normally an acid rinse of about 1 minute is sufiicient to remove residual oxides from the surface and condition the article for the application of a nickel flash coating.

The rinsed article is then washed with clean water to remove all traces of the sulfuric acid and is then provided with a nickel flash coating to obtain a nickel deposit of from 0.05 to 0.15 grams per square foot. A preferred solution for the application of the nickel flash coating is one containing from about A to .2 ounces per gallon of nickel sulfate and from A; to 1 ounce per gallon of sodium hypophosphate. The bath is operated at an elevated temperature of from about to F. Another suitable solution for providing a nickel flash coating is that described in Patent 2,977,241Oliver et al., and comprising a dilute solution of nickel sulfate adjusted by the addition of sulfuric acid and phosphoric acid to a pH of from about 2 to 3.5 and a phosphate ion content of from 0.001 to 0.03% by weight.

Following the nickel flash, the article is rinsed with clean water to remove the nickel solution and then rinsed with a dilute solution of soda ash having a pH of about 10.5 in order to neutralize any residues remaining on the surface of the article from the nickel flash treatment. Thereafter the article is dried and is ready for the application of a one-coat porcelain enamel such as that described in the above-mentioned Oliver et al. patent.

' In the initial preparation of the pickling bath, there is provided an aqueous solution of ferrous sulfate, ferric sulfate, sulfuric acid and nitric acid in the proportions required to provide a bath having the acid and iron sulfate concentrations given hereinbefore.

As has been previously indicated, the pickling bath of the present invention is characterized by the fact that it contains a dilute mixture of sulfuric and nitric acids within a certain range of proportions. These proportions along with the controlled ferric and ferrous ion contents of the pickling bath and a subsequent treatment of the article in a sulfuric acid bath have been found essential to provide a steel surface suitable for the subsequent application of the nickel fiash coating and the one-coat enamel. Unlike prior pickling baths containing higher concentrations of sulfuric or nitric acid, it is believed that the nitric acid in the subject bath functions only as an oxidizing agent for converting the ferrous to ferric ions and as such has no significant pickling effect on the steel surface. The actual etching or pickling action is believed to be caused by a reaction involving the ferric ions added to the solution or produced therein by action of the nitric acid on the ferrous ions. However, it has also been found that both the ferric and ferrous ion concentrations of the bath should be maintained within the limits indicated in order to obtain a controllable but adequate and effective conditioning of the steel surface. Minimum ferrous and ferric ion contents are apparently required for good bond of the enamel to the steel although a too high ferric ion content results in too much of an etching action and a tendency toward a pitted surface after enamelling.

The reactions involved in the pickling action are believed to be represented by the following equations:

As is seen from the first equation nitric acid is continuously consumed in the pickling operation and iron is continuously added to the solution by the pickling reaction of Equation 2. Also some of the sulfuric acid and sulfate ions are lost either by being carried over with the steel to the subsequent acid rinse bath or in the purging of some of the solution as hereinafter described to maintain the iron content of the bath below the upper limits. It is therefore necessary to continuously replace the sulfuric and nitric acids consumed in the pickling operation and at the same time to control the ferrous and ferric ion contents of the bath. For this purpose the iron concentration is controlled by periodically or continuously draining off some of the bath and replacing it with water. Also, a mixture of sulfuric and nitric acids in specific proportions is added to maintain the content of these two acids within the prescribed limits. A simple means for controlling the composition of the bath comprises the continuous addition of water to the bath so that it overflows its container thereby removing some of the iron salts. The water is added at a rate designed to maintain the bath within the specific gravity range of from about 1 to 1.08 at a temperature of 165 F. The sulfuric-nitric acid mixture is periodically or continuously added as required to maintain a pH of from 0.07 to 1.2.

In a series of experiments employing baths initially containing both ferrous and ferric sulfate or only one or the other, it has been found that both must be present and also that the ferric and ferrous ion contents of the bath are both critical. Unless there is some ferrous ion present it has been found that the pickling action of ferric ion content of the bath cannot be controlled. It appears that in the absence of ferrous sulfate, the ferric ion content continues to increase causing a continuously increasing rate of etch so substantial overflowing of the bath is required to keep the etching rate within the prescribed limits. Thus the process becomes more costly as larger quantities of acids are required to replace the acids lost as the bath is purged to maintain the ferric iron content below the maximum of 0.2% by weight.

In addition it has been found that the ratio of the sulfuric acid to nitric acid in the bath is also critical. A series of experiments bearing out this fact were conducted in which the 93% sulfuric acid and 40 B. nitric acid were added in the form of three different mixtures. In the first mixture, the added acids were mixed in the proportions of about 62 parts by volume of the sulfuric acid and 38 parts by volume of the nitric acid giving a nitricsulfuric acid weight ratio of about 1:33. In the second mixture the acids were mixed in the proportions of 55 parts of the sulfuric acid and 45 parts of the nitric acid giving a nitric-sulfuric acid ratio of 122.4. The third and preferred mixture contained 58 parts of the sulfuric acid and 42 parts of the nitric acid giving a nitric-sulfuric acid ratio of 1:2.8. In each case the acid mixtures were added at rates designed to maintain the pickling baths at pH values of between 0.8 and 1.0.

In each of these experiments, in order to limit the variable of overflowing or disposing of part of the solution in order to control the iron ion content, the iron content and specific gravity of the solutions were allowed to build up. The etch rates were maintained as close as possible to about 3 grams per square foot.

Employing the first acid mxiture containing 62% by volume 93% sulfuric acid and 38% by volume of 40 B. nitric acid, it was found that the enamel bond was not completely satisfactory and the enamel surface was below standard. Approximately 9.4 liters of the acid mix was consumed for the treatment of each 1000 square feet of steel surface while in the subsequent sulfuric acid rinse about 4.8 liters of the acid was consumed for the rinsing of each 1000 square feet.

Employing a 5545 mixture of the sulfuric and nitric acids in the bath, the bath was found to be too rich in nitric aicd and the etch rate was difficult to control tended to be higher than the desired rate. With this acid mixture approximately 12.5 liters of the acid mix was consumed for each 1000 square feet of steel surface while in the subsequent sulfuric acid rinse about 5.3 liters was consumed for the rinsing of 1000 square feet.

The third and preferred acid mixture containing about 58% of the sulfuric acid and 42% of the nitric acid was found to give a much lower acid consumption rate than either of the others. In other words, the rate at which the mixed acids had to be replenished was substantially lower. Employing this acid mixture, the mixed acids were consumed at the rate of only 3.3 liters per 1000 square feet of pickled surface while the sulfuric acid in the rinse was consumed at the rate of 7.6 liters per 1000 square feet. Thus the mixed acid consumption was only about one-third that obtained with the mixes which were higher or lower in sulfuric acid proportions While the consumption of sulfuric acid in the rinse was somewhat more but less than double the amounts.

The increased consumption of sulfuric acid in the rinse bath when using the 58-42 acid mix in the pickling bath and the improved results obtained thereby appears to indicate that some iron oxide does and should remain on the steel as it leaves the etch bath and that this iron oxide is converted to ferrous sulfate in the sulfuric acid bath thereby accounting for the higher consumption of sulfuric acid in the sulfuric acid rinse. Comparing the overall results of these experiments, optimum results with a minimum consumption of acids in the two baths are obtained when the ratio of nitric to sulfuric acid in the etch bath is kept within the limits of about 1 part by weight of nitric acid to about 2.6 to 3.1 parts sulfuric acid.

The following is a specific example of one manner in which the present invention can be carried into effect.

Example Articles formed from a low carbon sheet steel were spray cleaned using a hot solution of sodium phosphate to remove the oils and drawing compounds adhering thereto. The parts were then thoroughly rinsed with clean water to remove any loose matter and also all traces of the alkaline cleaner. The pickling bath contained in a tank having a 1370 gallon capacity was initially prepared by adding liters of 58-42 sulfuric acid-nitric acid mix, pounds of ferrous sulfate and 50 pounds of ferric sulfate to sufiicient water to fill the tank. The bath was maintained at a temperature between 165 F. The amounts of ferrous and ferric sulfate added were such that the bath initially contained about 0.1 percent ferric ion and 0.2 percent ferrous ion in the form of ferric and ferrous sulfates. In the production use of this bath for the pickling of steel articles, water was added to the bath at a rate of about 1 gallon per minute and the bath allowed to overflow the tank in order to maintain the iron content of the bath Wtihin a range such that the specific gravity of the solution was about 1.05 at F. A 58- 42% by volume mixture of the above-mentioned sulfuric and nitric acids was gradually added at the rate of about 200 milliliters per minute to hold the pH of the bath within the range of about 0.8 to 1.0. A 30 second spray of the steel articles with this bath was found to be suflicient to provide a pickling action such that about 3.0 grams iron per square foot of exposed surface was removed.

The pickled articles were immediately transferred to a sulfuric acid spray rinse containing 18 liters of 93 B. sulfuric acid per 300 gallons of solution. The sulfuric acid rinse was not heated although its temperature normal- 1y ran somewhat above room temperature and up to about 130 F. The normal drag out of the sulfuric acid rinse by the articles being removed from the rinse required a periodic or continuous replenishing of the bath by the addition of water and sulfuric acid to maintain the pH thereof between about 0.7 and 0.9. The articles were subjected to the sulfuric acid rinse for about 1 minute before being removed and flushed with clean water to remove all traces of the sulfuric acid and any sulfate adhering to the articles.

Thereafter the articles were sprayed for about 3 minutes with a nickel flash solution containing about 1 ounce per gallon of nickel sulfate and about 0.04 ounce per gallon of sodium hypophosphate. This nickel flash bath was operated at a temperature of about 140 F. and the pH was maintained between 3.0 and 3.5 normally without corrections. Whenever the PH tended to go below 3.0, caustic soda was added as required for control. Following the nickel flash treatment, the parts were rinsed with clean water to remove the nickel solution residue and were then sprayed with a neutralizing solution containing about 0.5% by weight of soda ash to neutralize the surface. All traces of the neutralizing solution were thereafter removed by a clean water rinse before the parts were allowed to dry. A one-coat enamel slip was then applied and fired to produce an enamel finish coat.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of treating a steel surface preparatory to the application of a nickel flash coating followed by a single finish coat of enamel which method comprises the steps of cleaning said surface in an aqueous alkali cleaning bath, pickling the cleaned surface in a hot aqueous pickling bath containing, by weight, from 0.5 to 2.0 percent sulfuric acid and from 0.2 to 1.0% nitric acid and having a controlled ferric and ferrous ion concentration equivalent to 0.1 to 0.5 percent by weight ferric iron and 0.2 to 3 percent by weight of ferrous iron to remove from the surface of said article from 1.5 to 3.0 grams of iron per square foot of exposed surface, and thereafter rinsing the pickled surface with an aqueous solution of from 0.5 to 5.0 percent by weight of sulfuric acid, said pickling bath being maintained at a specific gravity of about 1.0 to 1.08 at F. by the replacement of portions of said bath with water and at a pH of from .7 to 1.2 by the addition of sulfuric acid and nitric acid in the proportions of about 2.8 parts by weight of sulfuric acid to 1 part by weight of nitric acid.

2. The method of providing a steel surface with a single finish coat of enamel which method comprises the steps of cleaning said surface in an aqueous alkali cleaning bath, pickling the cleaned surface by means of a hot aqueous pickling bath containing, by weight, from 0.5 to 2.0 percent sulfuric acid and from 0.2 to 1.0% nitric acid and having a controlled ferric and ferrous ion concentration equivalent to 0.1 to 0.5 percent by weight ferric sulfate and 0.2 to 2 percent by weight of ferrous sulfate to remove from the surface of said article about 1.5 to 4 grams of iron per square foot of exposed surface, the sulfuric and nitric acid contents of said pickling bath being maintained by the addition thereto of said acids in the ratio of about 1 part by weight of nitric acid and 2.6 to 3.1 parts by weight of sulfuric acid, subjecting the pickled surface to the action of an aqueous solution of from 0.5 to 5.0 percent by weight of sulfuric acid, water rinsing said surface, applying a nickel flash coating to said rinsed surface, drying said article and thereafter applying a porcelain enamel slip and firing the slip to produce a one-coat enamel finish on said article.

References Qiteal in the file of this patent UNITED STATES PATENTS 2,411,532 Escoffery Nov. 26, 1946 2,569,453 Chester et al. Oct. 2, 1951 2,638,410 Zelley May 12, 1953 2,837,443 Zander June 3, 1958 2,872,302 Bulan Feb. 3, 1959 2,886,420 Jones et al. May 12, 1959 2,977,241 Oliver et al Mar. 28, 1961 3,078,180 Zander et al Feb. 19, 1963 

1. THE METHOD OF TREATING A STEEL SURFACE PREPARATORY TO THE APPLICATION OF A NICKEL FLASH COATING FOLLOWED BY A SINGLE FINISH COAT OF ENAMEL WHICH METHOD COMPRISES THE STEPS OF CLEANING SAID SURFACE IN AN AQUEOUS ALKALI CLEANING BATH, PICKLING THE CLEANED SURFACE IN A HOT AQUEOUS PACKLING BATH CONTAINING, BY WEIGHT, FROM 0.5 TO 2.0 PERCENT SULFURIC ACID AND FROM 0.2 TO 1.0% NITRIC ACID AND HAVING A CONTROLLED FERRIC AND FERROUS ION CONCENTRATION EQUIVALENT TO 0.1 TO 0.5 PERCENT BY WEIGHT FERRIC IRON AND 0.2 TO 3 PERCENT BY WEIGHT OF FERROUS IRON TO REMOVE FROM THE SURFACE OF SAID ARTICLE FROM 1.5 TO 3.0 GRAMS OF IRON PER SQUARE FOOT OF EXPOSED SURFACE, AND THEREAFTER RINSING THE PICKLED SURFACE WITH AN AQUEOUS SOLUTION OF FROM 0.5 TO 5.0 PERCENT BY WEIGHT OF SULFURIC ACID, SAID PICKLING BATH BEING MAINTAINED AT A SPECIFIC GRAVITY OF ABOUT 1.0 TO 1.08 AT 165*F. BY THE REPLACEMENT OF PORTIONS OF SAID BATH WITH WATER AND AT A PH OF FROM .7 TO 1.2 BY THE ADDITION OF SULFURIC ACID AND NITRIC ACID IN THE PROPORTIONS OF ABOUT 2.8 PARTS BY WEIGHT OF SULFURIC ACID TO 1 PART BY WEIGHT OF NITRIC ACID. 